The present invention relates to an ink jet printing apparatus wherein projects out from orifice an ink droplet and thereby prints letters and diagrams on the printing medium.
As for the ink jet printing apparatus, various types of them have been devised and put to practical use. As an example, the one of a drop-on-demand system shown in FIG. 1 can be given. In FIG. 1, 1 is an ink tank, 2 is a printing head and 3 is a common ink chamber that receives a supply of ink from the ink tank 1. Further, 4a, 4b, . . . , and 4g are the pressure chambers whose ends are connected to the common ink chamber 3 and 5a, 5b, . . . , 5g are the group of nozzles that project ink in the pressure chambers 4a, 4b, . . . , 4g respectively. On the flexible walls of aforesaid pressure chambers 4a, 4b, . . . , 4g, the piezoelectric-crystal elements (unillustrated) are arranged respectively in a manner of one element on one flexible wall and given piezoelectric-crystal element or elements are driven by the drive signals from the piezo driving section (unillustrated) and thereby ink droplets are projected out the shrunk pressure chambers 4a, 4b, . . . , and 4g through nozzles 5a, 5b, . . . , and 5g.
In addition, as the other method, the one called the continuous projecting system shown in FIG. 2 is available. In this apparatus, ink in an ink tank 11 is pressurized by a pump 12 to be supplied to an ink chamber 13 in a printing head 10. Owing to the vibration of a piezoelectric-crystal element 15 driven by the supplying pressure and a vibration source 14, the ink is continuously projected out from a nozzle 16 in the columnar form to become ink droplets. When the ink in the columnar form is separated into droplets, an electric charge corresponding to the image signal is given to the ink droplet by a charge electrode 17 and further, the flight direction of the droplet is controlled by a deflection electrode 18 and thus the image is formed.
In order to cause an ink droplet to fly correctly, high precision are requested on the shape and the surface condition of the nozzle and an ink path thereabout. There are various types of factors that prevent the correct projection and flight of the ink from the nozzle, and among those factors, there are given fine bubbles produced in the nozzle (hereinafter referred to as "bubbles" for abbreviation), clogging and dusts adhering to the outside of the nozzle and sticking into the nozzle as the one frequently occurs for the normal usage thereof. With these obstacles, there occur the phenomena wherein ink droplets can not be projected, or they can be projected but the speed thereof is abnormal or they do not fly straightly or the ink droplet splits and fly off in different direction. Namely, in the above examples, if bubbles enter the nozzles 5a, 5b, . . . , 5g and 16, or clogging in said nozzles takes place therein, the transmission of the pressure from the pressure chambers 4a, 4b, . . . , 4g or the common ink chamber 13 are prevented and further, the flow of the ink is prevented, thereby the ink droplets do not fly correctly.
Bubbles and clogging which are the causes for aforesaid obstacles are considered to happen for the following reasons.
As for the bubbles, first of all, the reasons are the following occasions: (1) the printing head receives a shock during the period of printing operation or the period of standby and owing to the abnormal acceleration caused by the above shock, the nozzle inhales bubbles, (2) when the drive signal is impressed on the piezoelectric-crystal element for the ink droplet to fly, the setting of the signal is impertinent or noises are superposed and the signal waveform is disturbed, thereby the nozzle inhales bubbles, (3) air dissolved in the ink separates out, or (4) during the preservation of the printing head, the surrounding temperature drops and the ink has a thermal contraction, thereby the nozzle inhales bubbles.
Clogging, on the other hand, takes place with ink in the nozzle drying and hardening when the printing head is left for a long time without being used or when the surrounding temperature drops abnormally. Clogging may further take place with duct or foreign substances in the ink condensing and adhering to the inside of the nozzle. Furthermore, the dust floating in the air or the paper dust from the recording paper adhere to the nozzle and enter the nozzle, thereby clogging takes place.
In order to remove bubbles and clogged substances in the nozzle which are obstacles for the normal flight of the ink droplet, there has been adopted a method in the conventional apparatus wherein the high pressure was impressed on the ink in the printing head from the ink supplying portion and thereby the bubbles and clogged substances in the nozzle are compulsorily swept away from the nozzle.
In the conventional method, however, the effect to remove the bubbles and clogged substances in the nozzle to the outside is not sufficient because the conventional method simply sweeps ink away and in many cases it was impossible to remove bubbles and clogged substances even if the ink in large amount was swept away repeatedly and was overflowed from the nozzle.
On the printing head wherein it is impossible to remove bubble or clogged substance in the nozzle, there has been taken a method wherein the ink is filled again after the ink in the head is removed, or, the print head is scrapped as condemned goods.